SPST 5V Relay (Hamlin HE3621A0500 RELAY REED SUBMIN SIP, Digikey HE203-ND) -
relay driving circuit
The relays don't always fit all the way into the boards because of little tabs on the pins.
Straightening and smoothing the pins with pliers before inserting them seems to solve it.
You can always test the relay once inserted by putting 5V across the coil and measuring continuity across the switch.